The present disclosure relates to optical communication technologies.
As the Internet, voice over Internet Protocol (VoIP), and Internet Protocol television (IPTV) grow in popularity, more and more users desire to have accesses to these services from their premises. The most common local network accesses to these services are the digital subscriber line (DSL) and the cable modem. The DSL and cable networks respectively operate on a pair of copper wires or coaxial cable. While the DSL and the cable modem allow data transfer at up to several million bits per second downstream to a user, the upstream data transfer is usually at lower transfer rate.
Passive optical network (PON) is attractive network architecture for the last-mile access because it does not require active components for directing optical signals between a central office and the network subscribers' terminal equipment. The PON can be divided into three main categories: time division multiplexing (TDM), wavelength division multiplexing (WDM), and a combination of TDM and WDM.
Fiber to the premises (FTTP) is a desirable architecture for providing access from the users' premises. FTTP takes optical fibers all the way into the users' homes or premises. Currently, time-division-multiplexing passive optical networks (TDM-PON) are the primary deployment methods for FTTP. TDM-PON is a point-to-multipoint architecture utilizing an optical power splitter at a remote node. TDM-PON delivers downstream information through broadcasting and bandwidth sharing, and receives upstream information via time division multiple access (TDMA). One drawback with TDM-PON is associated with the low security of the broadcasted downstream signals. The complexity of the TDMA protocols also makes TDM-PON undesirable for the next generation user-centric high-speed, broadband services.
A recent development in the FTTP is PON based on wavelength division multiplexing (WDM) technology. FIG. 1 illustrates a conventional WDM-based optical network 100 that includes a pair of WDM filters 108 and 116 for multiplexing and de-multiplexing between an optical line terminal (OLT) 102 and an optical network unit (ONU) 104. The WDM filters 108 and 116 are connected by a feeder fiber 101. The optical line terminal (OLT) 102 can be coupled to a plurality of optical network units (ONUs) 104 via a remote node 106. Each subscriber at an ONU 104 is assigned a separate WDM channel, whereby the channels are routed by a passive WDM filter 116 at the remote node 106. The OLT 102 includes a WDM filter 108 coupled to a plurality of band separators 110-1 . . . 110-N. Each band separator 110-1 . . . 110-N is further coupled to a transmitter 112-1, 112-2 . . . or 112-N and a receiver 114-1, 114-2 . . . or 114-N.
The passive WDM filter 116 at the remote node 106 provides a plurality of channels that each channel terminates at one of the ONU 104. Each ONU 104 includes a band separator 118-1 . . . 118-N each coupled to a transmitter 120-1, 120-2 . . . or 120-N and a receiver 122-1, 122-2 . . . or 122-N. The transmitters 120-1 . . . 120-N at the ONUs 104 provide narrow-spectrum light sources for upstream signals in a band A. The transmitters 112-1 . . . 112-N at the OLT 102 provide narrow-spectrum light source for the downstream signals in a different wavelength band B. The transmitters 120-1 . . . 120-N and 112-1 . . . 112-N are typically narrow-spectrum distributed-feedback (DFB) laser diodes with various wavelength tuning and stabilization mechanisms.
The above described conventional WDM-based optical network includes several drawbacks. The wavelength-specific narrow-spectrum light sources such as distributed-feedback (DFB) laser diodes are complex to make and have limited range of wavelength tunability by adjusting the laser operating temperature. The ONU for each subscriber uses at least one such laser. A large number of the wavelength-specific narrow-spectrum light sources are thus required in the conventional WDM-based optical network 100, which typically contains 32 or 40 ONUs. All the DFB laser chips have to be customer designed to the specific wavelength range for each group of a few wavelength channels. The emission center wavelengths of the DFB laser sources typically need to be fine tuned individually by temperature controller using precision spectral instrument to match the ITU wavelength grid of each wavelength channel. The inventory and field installation can become very complex and unmanageable in large-scale deployment for access.
Furthermore, the shift in the narrow spectrum of a wavelength-controlled DFB laser diode relative to the narrow wavelength channels of the DWDM wavelength filter can significantly affect the signal transmission. For example, a fraction of a degree of temperature drift can drive the emission spectrum of a narrow-spectrum laser out of the clear pass band window of the wavelength channel and cause significant loss of transmission signal. The reliability of the precision-temperature-controlled narrow-spectrum lasers is therefore a challenge in WDM-PON applications.